Polysilanes crystallinity

Another fertile area of current interest is the synthesis of stable homocyclic polysilane derivatives.Typical examples are cyclo-(SiMe2)7, > (cyclo-Si5Me9)-(SiMe2) -(cyclo-SisMeg), n = 2-5, and several new permethy-lated polycylic silanes such as the colourless crystalline compounds bicyclo[3.2.1]-Si8Mei4 (mp 245°), bicyclo[3.3.1]-Si9Mei6 (mp >330°) and... 

WAXD, DSC, and vibrational studies on //-hexyl-substituted polysilanes with 5%, 33%, or 67% /z-hexylsilyne branching points showed that the introduction of branching partially reduced the high-crystallinity characteristic of PDHS.365 It was also found that the charge carrier mobilities decreased with increasing branching points, and... 

The experimental observations of red shifts of the UV absorptions tails with increase in silicon dimensionality were corroborated by ZINDO-calculated spectra comparing linear polysilane, network polysilyne, crystalline cluster, and amorphous cluster structures, which showed respectively lowest absorption transition energies of 5.38 eV (230.4 nm), 4.60eV (269.5nm), 4.57eV (271.2nm), and 2.46eV (503.9nm), as shown in Figure 57.362... 

Twenty-five years later, Burhard reported the preparation of permethylated. polysilane (2). These materials were, however, highly crystalline, insoluble white solids which evoked little scientific interest until recently when it was discovered that silane polymers could be used as thermal precursors to / -silicon carbide fibers (3-5). In this regard, Yajima and co-workers reported that poly (dimethyl) silane could be converted by the two-step process shown below to / -silicon carbide, a structural material of considerable industrial importance. 

The observation that more soluble, less crystalline materials containing larger alkyl substituents (6) or aromatic groups (7,8) could be synthesized suggested additional applications for polysilane derivatives. In this regard, West and co-workers have reported that a soluble copolymer 1 produced by the... 

In recent years the literature has furnished a wealth of information concerning chemical structures from solid-state 29Si NMR studies of crystalline and noncrystalline silicates and aluminosilicate, polysiloxanes, polysilanes and other organosilicon compounds. 

The properties of the polysilanes, like those of the polyphosphazenes, depend greatly on the nature of the substituent groups. Polysilanes cover the entire range of properties from highly crystalline and insoluble, through partially crystalline, flexible solids, to glassy amorphous materials and rubbery elastomers. 

The properties of polysilanes depend very much on the nature of the organic groups bound to silicon. Some polymers are so highly crystalline as to be insoluble and... 

Both highly branched and cyclic permethylated polysilanes are snow-white crystalline solids with relatively high melting points. 

Tacticity is required for the synthesis of crystalline thin polysilane films used for optical and semiconductor devices. Modern synthetic routes allow control over the conformation and tacticity of polysilane molecules used as precursors for thin layers of photoresists, photoconductors and nonlinear optical phases in complex semiconductor and (opto)electronic devices. These properties can be exploited only if the synthesis method ensures a minimal level of contamination, especially with oxygen and metals, and special care is taken to limit electronic-grade polysilanes to a level of contamination on the order of a few ppm in the case of oxygen and in the ppb range for metals. The reactivity of polysilane toward oxygen has forced placing the devices in a helium environment during measurement procedures.36... 

Orbital energies of polysilanes are very sensitive to molecular conformation. Structured PDMS layers have been deposited by epitaxy on various substrates inducing various conformations via self-arrangement of the polymeric chains, either normal or parallel to the surface of the substrate, such as PTFE layers or freshly cleaved crystalline surfaces of KBr, NaCl, or KC1.5 Highly crystalline PDMS layers have been deposited on substrates and the UV absorption spectra are found to depend on the orientation of the crystal with respect to the substrate, as shown in Figure 6. 

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